Code Modernization: Replace isset() with null coalescing in WP_Roles::get_role().

Since PHP 7.0 introduced the [https://www.php.net/manual/en/migration70.new-features.php#migration70.new-features.null-coalesce-op null coalescing operator], and WordPress now requires at least PHP 7.2.24, `isset( $var ) ? $var : null` ternary checks can be safely replaced with the more concise `$var ?? null` syntax.

As some new code using the null coalescing operator has already been introduced into core in recent releases, this commit continues with the code modernization by implementing incremental changes for easier review.

Follow-up to [2703], [61403].

Props dilipbheda, mukesh27, spacedmonkey, SergeyBiryukov.
Fixes #63216. See #58874.

git-svn-id: https://develop.svn.wordpress.org/trunk@61404 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/class-wp-roles.php

@@ -268,11 +268,7 @@ public function remove_cap( $role, $cap ) {
 	 * @return WP_Role|null WP_Role object if found, null if the role does not exist.
 	 */
 	public function get_role( $role ) {
-		if ( isset( $this->role_objects[ $role ] ) ) {
-			return $this->role_objects[ $role ];
-		} else {
-			return null;
-		}
+		return $this->role_objects[ $role ] ?? null;
 	}
 
 	/**